PASS Scripta Varia 21 - Pontifical Academy of Sciences

MICHAEL HELLER
mological constant. And today there are strong reasons to believe that this
constant has an important physical interpretation. From recent observations
of the Ia type supernovae we know, with a good degree of credibility, that
the universe not only expands, but also accelerates its expansion. And to
obtain agreement between these observations and theoretical models one
should employ the equations with the cosmological constant (see below).
This is a typical story. The history of relativistic astrophysics and relativistic
cosmology from Einstein up to about the seventies of the previous
century, consisted mainly in solving Einstein’s equations, analyzing and interpreting
the structures of these solutions, and then looking at the sky to
verify what the equations had predicted (of course, it is a highly idealized
picture). In this process, various groups of people were engaged, there were
many disputes and erroneous interpretations, in some of them other departments
of physics had to be involved. In this way, young relativistic cosmology
slowly crystallized. The new field of research was created, still full
of question marks but ready for further developments.
Einstein’s paper of 1917 was definitely wrong. Its static world model
does not represent the world we live in. But the paper was epoch making;
it opened a new way in our thinking of the universe.
4. Domination of Philosophy
The next move belonged to the Dutch astronomer Willem de Sitter.
The story is well known; I shall only very briefly sketch its main stages. Yet
in 1917 de Sitter found another cosmological solution of Einstein’s equations
which represented a world devoid of matter (with vanishing matter
density) and, as it turned out later, with expanding space, thus ruining Einstein’s
hope that the ‘cosmological problem’ would have a unique solution.
In 1922 and 1924, Russian mathematician and meteorologist Alexander
Alexandrovitch Friedman published two papers in which he presented two
infinite classes of solutions with the positive and negative curvature of space,
respectively. In 1927 Georges Lemaître, for the first time, compared theoretical
predictions of one of the expanding cosmological models with the
results of galactic red shift measurements, and found that there was no contradiction
between them. All so far considered solutions satisfied the postulate
of maximal spacial symmetry (isotropy and homogeneity of space). 9
Mathematical discussion of such models was undertaken by Howard Percy
9
This postulate was called by Edward Arthur Milne the Cosmological Principle.
340 The Scientific Legacy of the 20 th Century